THE MYZOSTOMATA. 537 



other limbs, they are converted into the jaws of the adult 

 animal. It is remarkable that the antennas are developed 

 from the anterior part of the procephalic lobes ; while the 

 chelicerae of the Scorpion appear at the posterior margin of 

 these lobes, in a position corresponding with that of the first 

 pair of limbs, or jaws, of Peripatus. 



It is obvious that whether we consider the appendages, 

 the respiratory and reproductive systems, or the development 

 of the embryo, Peripatus is a true Arthropod, apparently 

 nearly allied to the suctorial Myriapoda. 



The Myzostomata. — The genus 3£yzostomum 1 compre- 

 hends certain small animals, the largest species not exceeding 

 one-fifth of an inch in length, which are parasitic upon the 

 Feather-stars. The body has the form of a flattened oval disk, 

 the surface of which is ciliated, while its margins may be 

 produced into as many as twenty short filamentous processes 

 or cirri. Within the margin of the ventral face are eight 

 suckers, four on each side, and, internal to these again, are 

 ten short conical " feet," five on each side ; each of these 

 lodges two strong seta?, which can be protracted and re- 

 tracted in the same way as those of Annelids. Just within 

 the middle of the anterior margin lies a rounded aperture, 

 through which a muscular proboscis, the free end of which is 

 beset with papillae, can be protruded. A straight alimentary 

 canal runs through the body, and terminates in a sort of 

 cloaca, which opens in the middle line on the posterior mar- 

 gin. From each side of the alimentary canal long ramified 

 cseca are given off. 



No vessels or organs of circulation have been discovered. 

 All that is known of the nervous system is an elongated gan- 

 glionic mass, from which branches are given off on each side, 

 situated in the middle line of the ventral face of the body. 



The sexes are combined in the same individual. The 

 acini of the generative glands are scattered through the body. 

 Those of the testes pour their contents into ducts, which 

 unite together and open by a separate vas deferens on each 

 side of the body, about the middle of its ventral face. The 

 two oviducts convey the ova to the cloacal chamber. 



The development of Myzostomum has been worked out 

 by Semper and by Metschnikoff. 2 The vitellus undergoes 



1 See Loven, " Archiv fur Naturgeschichte," 1842. 



2 Semper, " Zur Anatomie und Entwickelungsgeschichte der Gattung My- 

 zostomum." (Zeitschrift fur wiss. Zoologie, 1875.) "Zur Entwickelungsge- 

 schichte von Myzostomum." (Ibid., 1866.) 



